1. Field of the Invention
The present invention relates to a manufacturing method and a bonding apparatus for a liquid crystal display (LCD) device and, more particularly, to a manufacturing method and a bonding apparatus for a liquid crystal display device in a cell assembly step and a liquid crystal injection step.
2. Description of Related Art
In a cell assembly step, in a manufacturing process of a liquid crystal display device (hereinafter referred to also as “LCD”), a manufacturing method known as one drop fill (hereinafter referred to as “ODF”) has recently been receiving attention. According to the ODF method, a sealant and a liquid crystal are applied to an array substrate before bonding the array substrate with a color filter substrate. This allows the ODF method to reduce the time required for injecting a liquid crystal in a conventional cell assembly process. In other words, the ODF method leads to a significantly reduced time for the liquid crystal injection step, which results in a considerably shorter cell assembly timeline, whereas the conventional cell assembly process requires a lengthier period for injecting a liquid crystal.
FIG. 8 is a flowchart illustrating a cell assembly process including the ODF. Referring to FIG. 8, the cell assembly process includes a liquid crystal (LC) alignment step (S1), an ODF step (S2) and a polarizer bonding step (S3). In the LC alignment step S1, the array substrate (having an LC alignment layer applied thereto) and a polyimide (PI) film on the surface of the color filter substrate are rubbed in a predetermined direction by a rubbing roll so as to create micro grooves. Liquid crystal molecules are arranged in the predetermined direction along the micro grooves.
The ODF step S2 includes a sealant applying step S21, a liquid crystal applying step S22, a bonding step S23, a pressing step S24, a ultraviolet curing step S25 and an annealing step S26.
In the sealant applying step S21, a sealant 2 is applied in a rectangular frame on an array substrate 1 to match the shape of a cell, as shown in FIG. 9. In the liquid crystal applying step S22, a specified amount of a liquid crystal 3 is applied within the sealant 2 on the array substrate 1 by a dispenser. In the bonding step S23, the array substrate 1 and a color filter substrate disposed to oppose the array substrate 1, are bonded to each other in a vacuum.
At the time of bonding the two substrates together, the substrates are positioned such that the individual pixels of the array substrate 1 overlap the three primary colors of the color filter substrate. In the pressing step S24, the array substrate 1 and the color filter substrate bonded to each other in step S23 are contact-bonded by pressing them together. In the ultraviolet curing step S25, ultraviolet rays are applied to the bonded substrate assembly to cure the sealant. In the annealing step S26, isotropic processing for realigning the liquid crystal is carried out. The annealing heat fully cures the sealant 2. In the polarizer bonding step S3, a polarizer is attached to each of the two bonded substrates.
The LCD is fabricated by carrying out the steps of the cell assembly process described above, among which the bonding step is important. In the bonding step, in particular, the two substrates must be accurately positioned and the liquid crystal (after bonding the substrates together) must be free of bubbles. To prevent bubbles from remaining in the liquid crystal, the substrates are conventionally bonded in an intermediate vacuum atmosphere of about 10 Pa. Meanwhile, accurate positioning requires proper selection of a mechanism for holding the substrates. Currently, there are three available methods for holding an array substrate or a color filter substrate, a mechanical method, a vacuum holding method and an electrostatic holding method.
In the mechanical method, a physical means, such as a hook, is used to hold a substrate by securing its outer periphery. As the size of a substrate increases in recent years, the substrate tends to bend more from its own weight when it is retained by the mechanical method. This makes it difficult to achieve higher accuracy of positioning.
In the vacuum holding method, a substrate is attracted by creating a vacuum-like gap between a chuck and a substrate. The chuck used in the vacuum holding method features higher positioning accuracy and less expensive, compared with other methods. The vacuum holding method, however, is disadvantageous in that a substrate cannot be securely held because a pressure difference cannot be obtained between the pressure of the atmosphere and the pressure at which the chuck attracts the substrate when the bonding step is carried out in an intermediate vacuum atmosphere in order to prevent bubbles from remaining in a liquid crystal.
In the electrostatic holding method, a chuck for holding a substrate is electrostatically charged to hold the substrate by Coulomb force. Hence, a substrate can be retained even in a vacuum.
The positioning accuracy of the electrostatic holding method and the vacuum holding method are approximately the same; however, the electrostatic holding method is more extensively used often as substrates are bonded in a vacuum to prevent bubbles from remaining in a liquid crystal. The electrostatic holding method, however, may cause electrostatic damage to a substrate because a voltage of a few thousand volts is applied to retain a substrate. Furthermore, loading and unloading a chuck takes time, so that the bonding step takes time. There is another disadvantage in that the chuck used in the electrostatic holding method is extremely expensive.
Japanese Patent documents 1 through 3 (presented below) disclose a method in which a chucking apparatus uses the vacuum holding method in place of the electrostatic holding method. Patent document 1 discloses the use of a vacuum holding type chuck for improved positioning accuracy, but does not refer to any measures against remaining bubbles. Japanese Patent documents 2 and 3 disclose methods wherein remaining bubbles are prevented by changing the air pressure of the atmosphere in which the bonding step is carried out; this, however, requires a time to change the air pressure, undesirably leading to a longer bonding time.                Patent document 1: Japanese Unexamined Patent Application Publication No. 11-231328        Patent document 2: Japanese Unexamined Patent Application Publication No. 2000-258777        Patent document 3: Japanese Unexamined Patent Application Publication No. 2001-330840        Patent document 4: Japanese Unexamined Patent Application Publication No. 2002-284295        Patent document 5: Japanese Unexamined Patent Application Publication No. 2002-137352        
Unfortunately, despite these efforts, a suitable manufacturing method and a bonding apparatus for a liquid crystal display device that allow two substrates to be bonded to each other while expelling bubbles out from a liquid crystal remains desired.